I performed an audit the other day and one of the homeowners' largest concerns was a funky smell in the ducts that serve the upper stories (of a 3-story row home in Washington, DC). The smell is similar to that you will find in a bouse house (i.e. porta-John).
The smell came while running the heat.
The HVAC unit for the upper stories is an air-source heat pump.
The heat pump and ducts were recently installed by the guy who was flipping the house. An HVAC permit was not pulled.
Any ideas on what may be happening?